Oxygen removal

ABSTRACT

A process for reducing free oxygen in a gaseous nitrogen stream includes the steps of (i) reforming a hydrocarbon to generate a gas mixture containing hydrogen and carbon oxides, (ii) mixing the gas mixture with a nitrogen stream containing free oxygen, and (iii) passing the resulting nitrogen gas mixture over a conversion catalyst that converts at least a portion of the free oxygen present in the nitrogen to steam wherein the hydrocarbon reforming step includes oxidation of a hydrocarbon using an oxygen-containing gas.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 12/305,699 filed Dec. 19, 2008 which is the U.S. National Phaseapplication of PCT International Application No. PCT/GB2007/050261 filedMay 14, 2007, and claims priority to British Application No. 0612092.7filed Jun. 19, 2006.

FIELD OF INVENTION

This invention relates to a process for removing free oxygen fromnitrogen, in particular the removal of oxygen from nitrogen used tostrip or remove oxygen from another stream.

BACKGROUND OF INVENTION

GB 2127711 describes a process for degassing water using a recirculatinginert gas, such as nitrogen, which is regenerated and purified, in thegaseous state. The water is preferably seawater that is to be used asinjection water in underground oil reservoirs to obtain a higher degreeof hydrocarbon recovery. After contacting the water with the nitrogengas, the free oxygen transferred to the nitrogen gas is catalyticallyreacted to form steam. The catalytic reaction is performed in aconversion vessel filled with dry, granulated catalyst such as Pd or Pton alumina over which is passed the free oxygen-containing stripper gasand pure (99.9%) hydrogen. The pure hydrogen is provided by a waterelectrolyzer.

EP 0234771 describes an adaptation of this process using a plurality oftreatment stages each including a treatment zone through which arepassed in co-current flow the water and inert gas. Whereas pure hydrogenmay be used for catalytically converting free oxygen in the inert gasinto steam, because heat exchange means may be provided to heat thereductant, other reductants may be used such as methanol or natural gas,in particular methanol.

WO 2004/069753 describes yet another adaptation of this process whereina free oxygen containing inert stripping gas (nitrogen) is used tocombust dispersed hydrocarbons in the so-called “produced water”recovered in the production of oil & gas.

Using hydrocarbons to combust free oxygen in stripping gas requiresoperation of the catalyst at high temperatures, e.g. >300° C. Hightemperature operation of the catalyst is undesirable as it shortenscatalyst life and consumes large amounts of energy. Furthermore,catalyst poisons such as sulphur compounds are often present inhydrocarbon mixtures, especially those recovered from crude oil ornatural gas. Methanol combusts more cleanly but cannot generally beproduced locally to the de-gassing operation and so must be stored.Methanol is a toxic and highly flammable liquid and storage,particularly in off-shore operations, poses a number of technical andsafety difficulties. Electrolytic production of pure hydrogen remainsinefficient and expensive. Furthermore, local storage of hydrogen, e.g.in cylinders again poses a number of technical and safety problems.

Therefore there is a need to provide an efficient process for reducingthe free oxygen content of nitrogen stripping gas.

SUMMARY OF THE INVENTION

Accordingly the invention provides a process for reducing free oxygen ina gaseous nitrogen stream, comprising the steps of

-   -   (i) reforming a hydrocarbon to generate a gas mixture containing        hydrogen and carbon oxides,    -   (ii) mixing the gas mixture with a nitrogen stream containing        free oxygen, and    -   (iii) passing the resulting nitrogen gas mixture over a        conversion catalyst that converts at least a portion of the free        oxygen present in the nitrogen to steam wherein the hydrocarbon        reforming step includes oxidation of a hydrocarbon using an        oxygen-containing gas.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further illustrated by reference to the drawings inwhich:

FIG. 1 is a flowsheet of one embodiment of the process of the presentinvention; and

FIG. 2 is a flowsheet of a second embodiment of the process of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The hydrogen-containing gas mixture may be formed by partial oxidationof the hydrocarbon with an oxygen-containing gas, such as air, oxygen oroxygen-enriched air to produce a gas mixture containing hydrogen andcarbon monoxide as well as other gases such as unreacted C2+hydrocarbons, methane, carbon dioxide and nitrogen. Partial oxidation,may be carried out using any known partial oxidation process. Partialoxidation of a hydrocarbon may be performed by flame combustion in aburner using an oxygen-containing gas in the absence of a combustioncatalyst, by so-called non-catalytic partial oxidation (POx), orpreferably may be performed at lower temperatures in the presence of apartial oxidation catalyst by so-called catalytic partial oxidation(cPOx). In cPOx, the catalyst is preferably a supported Rh, Ni, Pd, orPt catalyst having <20% wt metal or alloy combinations of these metalson an inert support such as silica, alumina, titania or zirconia.

Alternatively, the hydrogen-containing gas mixture may be formed byautothermal reforming (ATR) comprising oxidising a portion of thehydrocarbon with an oxygen containing gas in the presence of steam togenerate carbon oxides and hydrogen, and steam reforming the resultinggas mixture containing unreacted hydrocarbon over a steam reformingcatalyst to produce a gas mixture containing hydrogen and carbon oxides.In autothermal reforming, therefore, steam is added with the hydrocarbonand/or oxygen-containing gas. The oxidation step, which may be performedcatalytically, is exothermic and generates the heat required by theendothermic steam reforming reactions. Nickel or precious metal steamreforming catalysts may be used. Precious metal catalysts are preferred.Precious metal catalysts used in reforming the hydrocarbon may includeone or more of Pt, Pd, Rh and Ir supported at levels up to 10% wt onoxidic supports such as silica, alumina, titania, zirconia, ceria,magnesia or other suitable refractory oxides, which may be in the formof pellets, extrudates, cellular ceramic and/or metallic monolith(honeycomb) or ceramic foam. In a preferred embodiment, the oxidationand steam reforming reactions are catalysed, more preferably over thesame catalyst composition so that one catalyst provides both functions.Such catalysts are described in WO 99/48805 and include Rh or Pt/Rh on arefractory supports comprising Ce and/or Ce/Zr-containing mixtures. Theprocess may be operated at inlet temperatures in the range 250-550° C.and outlet temperatures in the range 600-750° C. depending on the amountof preheat and O₂:C:H₂O ratio, and pressures of up to typically about 3bar abs.

As well as combustion and steam reforming reactions, the water-gas-shiftreaction takes place over the reforming catalyst. Thus the reactionstaking place in an autothermal reformer, where the hydrocarbon comprisesmethane are;

CH₄+2O₂-->CO₂+2H₂O

CH₄+H₂O-->CO+3H₂

CO+H₂O-->CO₂+H₂

We have found that such oxidation processes deliver a convenient sourceof hydrogen and provide a hydrogen containing gas with essentially nofree-oxygen content, particularly suited to the process of the presentinvention.

Desirably the POx, cPOX or ATR reforming apparatus is compact. We havefound that reforming apparatus designed for fuel cell hydrogengeneration is particularly suited to the present invention due to itsrelatively small size. Suitable apparatus for autothermal reforming isdescribed in EP0262947 and Platinum Met. Rev. 2000, 44 (3), 108-111, andis known as the HotSpot™ reformer.

In a preferred embodiment, the reformed gas mixture containing hydrogen,steam and carbon oxides (CO and CO₂) is cooled and passed over awater-gas-shift catalyst that reacts carbon monoxide with steam toincrease the hydrogen content of the gas mixture according to thefollowing equation.

CO+H₂O<-->H₂+CO₂

The water-gas shift catalyst may be precious metal-based, iron-based orcopper-based. For example a particulate copper-zinc aluminalow-temperature shift catalyst containing 25-35% wt CuO, 30-60% wt ZnOand 5-40% Al2O3% may be used at temperatures in the range 200-250° C.Alternatively the water gas-shift catalyst may be Pt on ceria.Particulate Pt/TiO₂ catalysts may also be used.

It is desirable that any heat exchange and water-gas shift apparatus arecompact so as to facilitate off-shore as well as on-shore installation.

In particular, the reforming and shift stages may be combined in compacthydrogen-generation apparatus wherein a hydrocarbon and oxygen arecombined over a precious metal partial oxidation catalyst, which mayalso function as a catalyst for the stream reforming reactions, and theresulting reformed gas mixture cooled and passed over a suitablewater-gas shift catalyst. Cooling of the reformed gas mixture may beperformed using heat exchange means, such as cooling coils or tubes, orby direct injection of water.

In one embodiment the hydrogen generation apparatus comprises a vesselin which is disposed a supported precious metal reforming catalyst and aseparate supported water-gas shift catalyst with heat exchange meanssuch as a heat exchange tube or tubes through which a coolant may bepassed, disposed between the catalysts. The hydrocarbon is fed with anoxygen-containing gas and steam to the reforming catalyst whereoxidation and steam reforming reactions take place. The resultingreformed gas mixture containing hydrogen, carbon oxides steam and asmall amount of unreacted hydrocarbon is then cooled by the heatexchange coils and then passed over the water-gas shift catalyst toincrease the hydrogen content of the hydrogen-containing gas. The use ofhydrogen generation apparatus comprising both reforming and shiftcatalysts is preferred apparatus in that it is very compact and maytherefore readily be installed in off-shore as well as onshorefacilities such as oil production platforms.

Whether hydrogen formation is by ATR, POx or cPOx, with or without thewater-gas shift reaction, it may be desirable to cool the resulting gasmixture before contacting it with the conversion catalyst. Preferablythe temperature of the gas mixture is cooled to ≦100° C., morepreferably ≦50° C., before it is contacted with the conversion catalyst.The reformed gas mixture is preferably cooled to below the dew point sothat water is condensed from the mixed gases. The water may then berecovered using known separator technology. Cooling of the gas mixturemay be effected using known heat exchanger technology. For example thegas mixture may be cooled using water under pressure in high and mediumpressure steam generation. Where hydrogen production is by means ofautothermal reforming, the steam may conveniently be used in the ATRprocess. In a preferred embodiment, at least a portion of the condensedwater is converted into steam for an ATR process. This reduces therequirement for fresh make-up water, which is advantageous in offshoreinstallations.

The hydrogen-containing gas formed from the hydrocarbon is combined withthe nitrogen gas containing free oxygen and the resulting gas mixturepassed over the conversion catalyst in order to react the hydrogen withthe free oxygen to produce steam. Alternatively or additionally, theconversion catalyst may convert the free oxygen into carbon dioxide byreaction with any carbon monoxide present in the mixed gas stream.

These reactions may proceed according to the following equations;

½O₂+H₂-->H₂O

½O₂+CO-->CO₂

The conversion catalyst is preferably a supported Group 8 transitionmetal catalyst. For example the catalyst may comprise one or more of Co,Ni, Pt, Pd, Rh, Ir or Ru on an oxidic support such as alumina, titania,zirconia or silica. Stable polymer catalyst supports may also be used.Preferably the catalyst comprises Pt, Pd, Co or Ni on alumina, e.g. ≦5%wt Pd on alumina. The conversion catalyst may be in the form of a woven,nonwoven or knitted mesh, particulate a foam, monolith or coating on aninert support. The conversion of the free oxygen is preferably performedat ≦300° C., more preferably ≦200° C., most preferably ≦150° C., with aninlet gas temperature preferably <100° C., more preferably <50° C.

A portion of the hydrogen-containing gas may if desired be subjected toa step of hydrogen separation e.g. using suitable membrane technology,and the recovered hydrogen sent upstream, e.g. for hydrodesulphurisationpurposes.

In a preferred embodiment, the hydrocarbon is natural gas, i.e. amethane-rich gas stream containing minor amounts of C2+ hydrocarbons.The natural gas may be a “raw” natural gas as recovered fromsubterranean sources, including associated gas recovered with crude oil,or may be a “process” natural gas that has been used in a process, suchas a stripping gas. Natural gas liquids (NGLs) may also be used.

If desired, sulphur and or mercury or arsenic absorbers may be provided,e.g. upstream of the hydrogen forming step, to protect any catalystsused therein from poisoning. Suitable sulphur absorbers include zincoxide compositions, preferably copper-containing zinc oxide compositionswhereas mercury and arsenic are usefully absorbed on metal sulphidessuch as copper sulphide. Particularly suitable sulphur and mercuryabsorbents are described in EP0243052 and EP0480603. Additionally,hydrodesulphurisation may also be performed upstream of any adsorbentsusing known Ni or Co catalysts to convert organic-sulphur, -nitrogen-mercury and -arsenic compounds into more readily removable materialssuch as H₂S, NH₃, Hg and AsH₃.

Although upstream sulphur removal may be desirable to protect thedownstream catalysts, in cases where a precious metal reforming catalystis employed upstream of a copper-based water gas shift catalyst, it maybe desirable in addition or as an alternative to include a sulphurabsorbent between the reforming catalyst and water-gas shift catalyst.

In the present invention, the nitrogen containing free oxygen haspreferably been used to reduce the content of dissolved oxygen fromwater. The water may be a fresh-water, brine, seawater, produced water,cooling water or injection water.

Accordingly the invention further provides a process for the reducingthe free oxygen content of water comprising the steps

-   -   (i) contacting water containing dissolved oxygen with a nitrogen        stream to form a free-oxygen containing nitrogen stream and a        de-oxygenated water stream, and    -   (ii) reducing the oxygen content of said free-oxygen containing        nitrogen stream by        -   a. reforming a hydrocarbon to generate a gas mixture            containing hydrogen and carbon oxides,        -   b. mixing the gas mixture with a nitrogen stream containing            free oxygen, and        -   c. passing the resulting nitrogen gas mixture over a            conversion catalyst that converts at least a portion of the            free oxygen present in the nitrogen to steam            wherein the hydrocarbon reforming step includes the            oxidation of a hydrocarbon using an oxygen-containing gas.

Hence nitrogen containing free oxygen is preferably a nitrogen strippergas, which may be used in processes described in GB 2127711, EP 0234771or WO 2004/069753.

The water that is contacted with the nitrogen stream may be fresh-water,brine, seawater, produced water, cooling water or injection water. Thefree-oxygen levels in such waters may be 10 ppm or higher. Using theprocess of the present invention the free-oxygen levels may be reducedto 20 ppb or lower. In one embodiment, the deoxygenated water stream isused in an enhanced oil recovery process to recover a crude oil.

Preferably the hydrocarbon used as the source of hydrogen is ahydrocarbon recovered as part of a crude oil/natural gas productionprocess. Thus in a preferred process, a stream of hydrocarbon,preferably gaseous hydrocarbon, is separated from oil and gasproduction, used to form a hydrogen-containing gas mixture by ATR, POxor cPOx and this mixture, optionally following a step ofwater-gas-shift, combined with the nitrogen gas containing free oxygen.The hydrocarbon may be one containing free oxygen or one that has beencontacted with a nitrogen stripper gas and so is depleted in freeoxygen. The hydrocarbon is preferably a methane-rich hydrocarbon such asnatural gas or associated gas. The volume of hydrocarbon separated fromthe production stream is preferably only enough to generate sufficienthydrogen and/or carbon monoxide required to reduce the free oxygencontent of the nitrogen stripper gas to acceptable levels, e.g. to ≦5ppm. The amount withdrawn for oxidation is therefore preferably ≦5% byvolume of the gaseous hydrocarbon stream.

The apparatus used for the process of the present invention may beconveniently compact, in particular where hydrogen generation apparatuscomprising separate reforming and water gas shift catalysts is used. Ina particularly preferred embodiment, the process of the presentinvention comprises hydrogen generation using apparatus with separatereforming and water-gas shift catalysts combined with the so-calledMINOX™ processes for de-oxygenation of nitrogen stripper gases.

Thus in one embodiment, oxygen may be removed from nitrogen used in aprocess wherein two separators fed with a seawater and nitrogen mixtureare operated in series with nitrogen mixed with the seawater upstream ofeach separator and wherein the nitrogen stream fed to the firstseparator is the separated gas from the second separator. Thefree-oxygen containing nitrogen stripper gas leaving the first separatoris passed to a conversion vessel where the oxygen is reacted withhydrogen and/or carbon monoxide produced in the process of the presentinvention to generate steam and/or carbon dioxide. The nitrogen mixture,depleted in free oxygen, may then be fed to the second separator as thenitrogen stripper gas. As an alternative to the two-stage separatorprocess, a single compact tower and packing vessel may be used withsimilar effect.

For closed-loop water circuits, such as cooling water circuits, it isdesirable to operate the process until the desired oxygen level isachieved and then to operate the process only when needed, e.g., whenmake-up water is added or if the water circuit has been opened to theatmosphere for maintenance. Alternatively the process may be operatedcontinuously where “fresh” water containing dissolved oxygen isconstantly required, e.g. in enhanced oil recovery operations forinjection water or produced water.

The nitrogen used in the process of the present invention is preferablypure nitrogen but some oxygen may be tolerated initially as theconversion catalyst will react hydrogen with this to lower the freeoxygen content to acceptable levels. Air may be used as a top up gas aslong as sufficient hydrogen is available to satisfy the demand and theoperation of the process is controlled to prevent catalyst degradationby the exothermic conversion reaction.

The invention further provides apparatus for reducing the free oxygencontent of a nitrogen stream, comprising a conversion vessel havingfree-oxygen-containing gaseous nitrogen inlet means, a conversioncatalyst disposed within said vessel and local to said conversion vesselhydrogen generation apparatus that provide a hydrogen-containing gasoperatively connected to said vessel such that the nitrogen is mixedwith said hydrogen-containing gas and passed over said catalyst, whereinthe hydrogen generation apparatus comprises a reformer vessel that hashydrocarbon inlet means, oxygen-containing gas inlet means andoptionally a reforming catalyst disposed within said vessel.

The hydrogen generation apparatus may comprise an autothermal reformerhaving hydrocarbon and steam inlet means, an oxygen-containing gas inletmeans, and a steam reforming catalyst.

Alternatively, the hydrogen formation means comprise a partialcombustion vessel, having hydrocarbon and oxygen-containing gas inletmeans, and optionally containing a partial oxidation catalyst.

Preferably, a water-gas-shift catalyst is disposed downstream of saidreforming catalyst, more preferably downstream of said reformingcatalyst and disposed within the reformer vessel, especially withcooling means such as heat exchange tube or tubes between said reformingand water-gas shift catalysts.

Where inlet means are provided to a vessel, it will be understood thatsuitable product outlet means are also provided.

If desired, suitable heat exchanger means may be provided to cool thegaseous product stream from the hydrogen generation apparatus to preventdecomposition of the conversion catalyst.

Although the nitrogen has been described herein as a stripping gas forwater, other uses of the present technology are foreseen outside waterstripping, for example, liquid hydrocarbon or alcohol stripping.

In FIG. 1, a gas/oil mixture from oil production is fed via line 10 toseparator 12 where a portion of natural gas is separated from the oilwhich is recovered via line 14. The portion of natural gas is passedfrom separator 12 via line 16 to purification vessel 18, containing aparticulate copper-zinc oxide composition 20 that removes hydrogensulphide from the gas stream. The desulphurised gas is then preheated bymeans of a heat exchanger (not shown) and fed via line 22 to hydrogengeneration vessel 24 containing a monolithic Rh on Ceria-doped zirconiareforming catalyst 26. Such a catalyst catalyses both the combustion andsteam reforming reactions. The desulphurised gas is mixed with oxygenand steam fed to the hydrogen generation vessel 24 via line 28 and themixture autothermally reformed (oxidised and steam reformed) over thecatalyst 26. The reformed gas stream comprising hydrogen, steam andcarbon oxides, including carbon monoxide, is cooled by means of heatexchange tubes 29 within the vessel 24 downstream of the reformingcatalyst 26. The cooled gases then pass to a bed of low-temperatureshift catalyst 30 disposed within same vessel 24 downstream of said heatexchange tubes 29. The cooled gas mixture reacts over the catalyst 30 toincrease the hydrogen content of the gas mixture by the water-gas shiftreaction. The resulting hydrogen-enriched gas mixture is then passedfrom the hydrogen generation vessel 24 via line 32 to a heat exchanger34 where it is cooled. The gas is cooled to below the dew point tocondense water, which is recovered via line 35 by means of a separator(not shown) and may be used to generate steam for the autothermalreforming. The cooled gas stream containing hydrogen is then mixed witha free-oxygen containing gas stream fed via line 36 and thenitrogen/hydrogen-containing gas mixture passed via line 38 to aconversion vessel 40 containing a bed of supported precious metalconversion catalyst 42. The oxygen in the nitrogen reacts with thehydrogen in the mixed gas stream over the catalyst 42 to form steam,thereby depleting the nitrogen of free oxygen. The oxygen-depletednitrogen passes from conversion vessel 40 via line 44 to near the bottomof a stripping tower 46 containing a packing 48. The stripping tower 46is fed with water containing dissolved oxygen via line 50 near the topof the tower. The water is distributed over the top of the packing 48 bydistributor means 52 and the water passes down through the packing 48under the force of gravity. The oxygen-depleted nitrogen fed via line 44passes up through the packing counter-current to the descending waterand thereby contacts the water thereby depleting the water of dissolvedoxygen. The oxygen-depleted water is recovered from the bottom of tower46 via line 54 and may be used in enhanced oil recovery operations toproduce the oil/gas mixture 10. The nitrogen passing up through thepacking 48 picks up the dissolved oxygen and the resultingoxygen-containing nitrogen stream is conveyed from tower 46 via line 56and compressor 58 to optional heat exchanger 60 where it may be heatedbefore being fed via line 36 to be mixed with the cooledhydrogen-containing gas stream from heat exchanger 34. Air fed via line62 to line 56 between the tower 46 and compressor 58 may be used to topup the nitrogen gas stream.

In FIG. 2, the hydrogen generation and oxygen removal (from N₂) stagesare identical to those in FIG. 1. In this embodiment, theoxygen-depleted nitrogen passes from conversion vessel 40 via line 44and is mixed with partially deoxygenated water fed via line 70 from thebottom of a first separator 72. The water/nitrogen mixture is passed toa second separator 74. In the second separator 74 the nitrogencontaining free oxygen separates from the de-oxygenated water, which isrecovered from the bottom of the separator 74 via line 76. The nitrogencontaining free oxygen is recovered from the top of the second separator74 via line 78, mixed with water containing dissolved oxygen fed vialine 80 and the mixture fed to the first separator 72. The nitrogencontaining free oxygen recovered from the top of the first separator 72is conveyed via line 82 and compressor 58 to optional heat exchanger 60where it may be heated before being fed via line 36 to be mixed with thecooled hydrogen-containing gas stream from heat exchanger 34. Air fedvia line 62 to line 82 between the separator 72 and compressor 58 may beused to top up the nitrogen gas stream.

Using the process, the oxygen content of seawater may be reduced fromabout 9 ppm to less than 20 ppb.

What is claimed:
 1. An apparatus for reducing the free oxygen content ofa nitrogen stream, comprising a conversion vessel having gas inlet meansfor providing free-oxygen-containing gaseous nitrogen and ahydrogen-containing gas to said vessel, product gas outlet means, aconversion catalyst disposed within said vessel between said inlet andoutlet means and local to said conversion vessel, hydrogen generationapparatus that provides a hydrogen-containing gas, operatively connectedto said conversion vessel, such that the free-oxygen-containing nitrogenis mixed with said hydrogen-containing gas and passed over saidconversion catalyst wherein the hydrogen generation apparatus comprisesa reformer vessel that has hydrocarbon inlet means, oxygen-containinggas inlet means, reformed gas outlet means and optionally a reformingcatalyst disposed within said vessel between said inlet and outletmeans.
 2. The apparatus according to claim 1 wherein the hydrogengeneration apparatus comprises an autothermal reformer havinghydrocarbon and steam inlet means, an oxygen-containing gas inlet means,reformed gas outlet means and a reforming catalyst disposed between saidinlet and outlet means.
 3. The apparatus according to claim 1 whereinthe hydrogen generation apparatus comprises a partial combustion vessel,having hydrocarbon and oxygen-containing gas inlet means, reformed gasoutlet means and optionally containing a partial oxidation catalystdisposed between said inlet and outlet means.
 4. The apparatus accordingto claim 1 further comprising a water-gas-shift catalyst disposeddownstream of said reforming catalyst in the reformer vessel.
 5. Theapparatus according to claim 4 further comprising cooling means providedwithin the reforming vessel between the reforming catalyst and the shiftcatalyst that cool the reformed gas before it passes over the shiftcatalyst.